Minimum Genetic Mutation LeetCode solution in JavaScript

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/**
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 * @param {string} start
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 * @param {string} end
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 * @param {string[]} bank
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 * @return {number}
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 */
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var minMutation = function (start, end, bank) {
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  if (start.length != end.length || !contains(end, bank)) {
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    return -1;
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  }
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  return bfs(start, end, bank);
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};
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function contains(end, bank) {
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  for (const x of bank) {
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    if (x == end) {
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      return true;
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    }
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  }
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  return false;
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}
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function convertPossible(a, b) {
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  if (a.length != b.length) return false;
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  let count = 0;
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  for (let i = 0; i < a.length; i++) {
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    if (a[i] != b[i]) {
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      count++;
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    }
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  }
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  return count == 1;
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}
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function bfs(start, end, bank) {
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  let count = 0;
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  let q = [];
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  let set = new Set();
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  q.push(start);
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  while (q.length > 0) {
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    let size = q.length;
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    for (let i = 0; i < size; i++) {
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      let curr = q.shift();
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      if (curr == end) {
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        return count;
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      }
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      bank.forEach((x) => {
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        if (convertPossible(curr, x) && !set.has(x)) {
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          q.push(x);
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          set.add(x);
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        }
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      });
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    }
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    count++;
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  }
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  return -1;
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}